在以下smil编程中怎么样让play.RM视频文件只播放声音,而让play.rp播放图像
<smil>
<head>
<meta name="noname" content="empty" skip-content="true" />
</head>
<body>
<par endsync="id(vrm)" repeat="1">
<video id="vrm" region="videorm" src="play.rm" repeat="1"
clip-begin="8.1" clip-end="33:00.2"
fill="freeze" />
<ref id="pppt" region="pptshow" src="play.rp" repeat="1" fill="freeze" />
</par>
</body>
</smil>
